Heat treating metals involves altering their microstructure and properties through processes like annealing, normalizing, hardening, and tempering, which involve carefully controlling the heating and cooling of metals. Smaller crystal formation from rapid cooling, as in hardening, increases hardness but causes brittleness, while slower cooling encourages larger crystals that make metals softer and more ductile. Tempering a hardened metal by gentle reheating relieves internal stresses from hardening to increase toughness while maintaining hardness.

1. Heat Treating MetalsHeat Treating Metals
There can be few alterations so extremeThere can be few alterations so extreme
as to convert a relatively soft andas to convert a relatively soft and
ductile material into one which is hardductile material into one which is hard
enough to act as a cutting toolenough to act as a cutting tool
Tubal CainTubal Cain

3. Typical Exam Question
Mild steel is not suitable for hardening due to
the low content of carbon, the outer surface
however can be hardened.
1. Name the process for hardening the surface of
mild steel. (1 Mark)
2. Describe the advantage of hardening a component
in this way. (3 Marks)

4. RevisionRevision
An ATOM is the basic building block of materialsAn ATOM is the basic building block of materials
ATOMS arrange themselves into Molecules e.g.ATOMS arrange themselves into Molecules e.g.
Water or HWater or H22OO
Materials can exist in three statesMaterials can exist in three states
SOLID e.g. ICESOLID e.g. ICE
LIQUID e.g. WATERLIQUID e.g. WATER
GASS e.g. STEAMGASS e.g. STEAM

5. SOLID

6. LIQUID

7. GAS

8. Microstructure of common metalsMicrostructure of common metals
Steel Copper
Brass

9. Metal Crystal FormationMetal Crystal Formation
A – Nuclei FormA – Nuclei Form
B – Dendrites Start to GrowB – Dendrites Start to Grow
C – Fastest Growth in line with coolingC – Fastest Growth in line with cooling
D – Growing Crystals meet and create boundariesD – Growing Crystals meet and create boundaries
B – Upper Critical pointB – Upper Critical point
C – Lower Critical pointC – Lower Critical point

10. SteelSteel
• On the previous image s you could see theOn the previous image s you could see the
growth of the crystals in the metal & thegrowth of the crystals in the metal & the
boundaries that are formedboundaries that are formed
• These structures are known as the GrainThese structures are known as the Grain
• When the material is worked it can become workWhen the material is worked it can become work
hardened, internal stress built up due tohardened, internal stress built up due to
compaction of the crystal structure and thecompaction of the crystal structure and the
material also becomes more brittle.material also becomes more brittle.
• Lines of weakness/fracture can formLines of weakness/fracture can form

11. Heat TreatmentsHeat Treatments
• There are 4 common heat treatment processesThere are 4 common heat treatment processes
that we need to understandthat we need to understand
– NormalisingNormalising
– AnnealingAnnealing
– HardeningHardening
– TemperingTempering
• These can all be used successfully on commonThese can all be used successfully on common
work shop metals such as Steel, Copper & Brass.work shop metals such as Steel, Copper & Brass.

12. Normalising & AnnealingNormalising & Annealing
These are both very similar processes.These are both very similar processes.
Normalising is about producing uniform crystalNormalising is about producing uniform crystal
structure, which improves toughness andstructure, which improves toughness and
ductility and malleability.ductility and malleability.
Annealing is about releasing internal stressesAnnealing is about releasing internal stresses
due to working the material by allowing largedue to working the material by allowing large
crystal to grow, making the material softer andcrystal to grow, making the material softer and
again more ductile or malleable.again more ductile or malleable.
These are both achieved by heating the materialThese are both achieved by heating the material
to a point just above it’s critical point andto a point just above it’s critical point and
allowing it soak in this heat and then coolallowing it soak in this heat and then cool
slowly, usually in air.slowly, usually in air.

13. These are before and after shots ofThese are before and after shots of
Cold Worked CopperCold Worked Copper
Which was then annealed at 800 degrees centigradeWhich was then annealed at 800 degrees centigrade
Note the scale and the size of the crystals.Note the scale and the size of the crystals.

14. HardeningHardening
So if we allow a metal to soak in heat and thenSo if we allow a metal to soak in heat and then
cool slowly, so as to encourage large crystalscool slowly, so as to encourage large crystals
to grow and this makes the material, softer,to grow and this makes the material, softer,
more ductile and malleable.more ductile and malleable.
What happens if we heat the metal and then coolWhat happens if we heat the metal and then cool
it rapidly, say in water?it rapidly, say in water?
Smaller crystals form leading to improvedSmaller crystals form leading to improved
hardnesshardness in the metal but this also cause thein the metal but this also cause the
material to become more brittle due to thematerial to become more brittle due to the
internal stresses created.internal stresses created.

15. TemperingTempering
We know if we heat a metal up and then quench itWe know if we heat a metal up and then quench it
rapidly small crystals form, leading torapidly small crystals form, leading to
improved hardness in the metal but this alsoimproved hardness in the metal but this also
cause the material to become more brittle duecause the material to become more brittle due
to the internal stresses created.to the internal stresses created.
What happens if we just heat it gently and thenWhat happens if we just heat it gently and then
quench it?quench it?
So gently heating a metal allows some movement inSo gently heating a metal allows some movement in
the crystal structure, relieving internalthe crystal structure, relieving internal
stresses and reducing brittleness (increasingstresses and reducing brittleness (increasing
toughness) while not allowing larger crystalstoughness) while not allowing larger crystals
to form, so helping maintain hardness.to form, so helping maintain hardness.

16. It may appear to that all that is happening isIt may appear to that all that is happening is
that you are heating up a piece of metalthat you are heating up a piece of metal
and then dropping it in a bucket of tepid water.and then dropping it in a bucket of tepid water.
But it is this technology especially combinedBut it is this technology especially combined
with metals such as iron and steel that haswith metals such as iron and steel that has
revolutionised our world.revolutionised our world.
From the smallest metal artifactFrom the smallest metal artifact
To some of the largest structures ever builtTo some of the largest structures ever built

17. TESTTEST
1.1. Name a process for relieving internal stress in worked metal?Name a process for relieving internal stress in worked metal?
2.2. Name one disadvantage to hardening a metalName one disadvantage to hardening a metal
3.3. Why should ‘cold’ water not be used for quenchingWhy should ‘cold’ water not be used for quenching
4.4. After hardening what process can be used to reduce brittleness?After hardening what process can be used to reduce brittleness?
5.5. What causes the colour change during this processWhat causes the colour change during this process
6.6. Name a common use for each of these metalsName a common use for each of these metals
1.1. Mild SteelMild Steel
2.2. CopperCopper
3.3. BrassBrass
4.4. AluminiumAluminium